rate of reaction
DESCRIPTION
igcse chemistryTRANSCRIPT
7. How far? 7. How far? How fast?How fast?
7.1 Energy changes in 7.1 Energy changes in chemical reactionschemical reactions
Chemical reactions are capable of releasing vast amounts of energy.
Hydrocarbon molecules contain only the elements carbon and hydrogen.
Methane + oxygen → carbon dioxide + water
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
The reaction between methane and oxygen
During this reaction, as with all others, bond are first broken and then new bonds are made.
In methane molecules, carbon atoms are covalently bonded to hydrogen atoms. In oxygen gas, the atoms are held together in diatomic molecules.
During the reaction, all these bonds must be broken. Chemical bonds are forces of attraction between atoms or ions.
To break these bonds requires energy; Energy must be taken in to pull the atoms apart.
Breaking chemical bonds takes in energy from the surroundings. This is an endothermic process.
New bonds are then formed: between carbon and oxygen to make carbon dioxide, and between hydrogen and oxygen to form water.
Forming these bonds gives out energy.
Making chemical bonds gives out energy to the surroundings. This is an exothermic process.
When methane reacts with oxygen, the total energy given out is greater than the total energy taken in.
So, overall, this reaction gives out energy – it is an exothermic reaction. The energy is released as heat.
The overall change in energy for this exothermic reaction can be shown in an energy level diagram.
Exothermic ReactionExothermic ReactionReactants Products + Energy
Reactants
Products
-HEn
erg
y
Energy of reactants
Energy of products
Reaction Progress
Energy/ kJ CH4 (g) + 2O2
(g)
CO2 (g) + 2H2O (g)
Heat given out
Progress of reaction
In this reaction, energy is given out because the bonds in the products (CO2 and H2O) are stronger than those in the reactants (CH4 and O2)
This means that the products are more stable than the reactants.
Some bonds are stronger than others. They require more energy to break them, but they give out more energy when they are formed.
The combustion reactions of fossil fuels such as oil and gas are exothermic.
The major characteristics that make these fuels so useful are that: • they are easy to ignite and burn•They are capable of releasing large amounts of energy as heat.
Endothermic reactions are far less common than exothermic ones.
The reaction between nitrogen and oxygen
Here, energy is absorbed from the surroundings.
The reaction between nitrogen and oxygen is endothermic.
It is one of the reaction that take place when fuel is burnt in car engines.
Nitrogen + oxygen → nitrogen monoxide
N2 (g) + O2 (g) → NO (g)Here the bonding in the products is weaker than in the reactants. Overall, energy is taken in by the reaction.
The energy change in going from reactants to products in a chemical reaction is known as the heat of reaction.
Heat of reaction
It is given the symbol ∆ H ( the symbol ∆ means ‘change in”
The energy given out or taken in is measured in kilojoules (kJ);1 kJ = 1000 J.
Heat of reaction
It is usually calculated per mole of a specific reactant or product (kJ / mol)
The starting point for the calculation is the reacting mixture. If a reaction gives out heat to the surroundings, the mixture has lost energy.
Exothermic reaction
It is an exothermic reaction.
In EXothermic reaction, heat EXits the reaction mixture.An exothermic reaction has a negative value of ∆ H.
Exothermic reaction
Exothermic ReactionExothermic ReactionReactants Products + Energy
Reactants
Products
-HEn
erg
y
Energy of reactants
Energy of products
Reaction Progress
If a reaction takes in heat from the surroundings, the mixture has gained energy.
Endothermic reaction
It is an endothermic reaction.
In ENdothermic reaction, heat ENters the reaction mixture.An endothermic reaction has a positive value of ∆ H.
Exothermic reaction
Endothermic ReactionEndothermic ReactionEnergy + Reactants Products
+H
Reaction progress
En
erg
y
Reactants
ProductsActivation Energy
Heat of reaction: for exothermic reactions, heat
energy is given out (exits) and ∆H is negative
for endothermic reactions, heat energy is taken in (enters) and ∆H is positive
These ideas fit with the direction of the arrows shown in the energy diagrams.
Energy/ kJ CH4 (g) + 2O2
(g)
CO2 (g) + 2H2O (g)
Heat given out (-
728kJ/mol)
Progress of reaction
Exothermic reaction
Experiments have been carried out to find out how much energy is needed to break various covalent bonds in compounds.
Making and breaking bonds
The average value obtained for a particular bond is known as the bond energy.It is a measure of the strength of the bond.
Energy/ kJ
N2 (g) + O2 (g)
2NO2 (g)
Heat taken in
Progress of reaction
ENERGY LEVEL DIAGRAMSENERGY LEVEL DIAGRAMS• We can show the energy transfers in
reactions on an energy level diagram.
• These show us the energy stored in the reactants compared to the energy stored in the products.
Exothermic energy level diagramExothermic energy level diagram
Exothermic energy level diagramExothermic energy level diagram• ∆H (‘delta H’) is the
symbol for the ‘change in energy’.
• In an exothermic reaction the products have less energy than the reactants.
• ∆H is negative for an exothermic reaction.
Endothermic energy level diagramEndothermic energy level diagram
Endoothermic energy level Endoothermic energy level diagramdiagram
• In an endothermic reaction the products have more energy than the reactants.
• ∆H is positive for an endothermic reaction.
Making and breaking bondsMaking and breaking bonds• We have seen how bonds are formed
(both ionic and covalent)• Do you think energy is needed to
break bonds?
• Breaking bonds requires energy. It is endothermic.
• Breaking bonds requires energy. It is endothermic.
• Making new bonds gives out energy. It is exothermic.
Heat of combustion
The energy change of a reaction when a substance is burnt.
Heat of neutralisation
The energy change of a reaction when an acid react with an alkali to form water.
•the amount of energy that is required to start a chemical reaction.
•Once activation energy is reached the reaction continues until you run out of material to react.
.
Activation energy
∆H
•More usually, energy is required to start the reaction.
•When fuels are burnt, for example, energy is needed to ignite them.
Activation energy
•This energy may come from a spark, a match or sunlight.
•It is called the activation energy (given the symbol EA)
•It is required because initially some bonds must be broken before any reaction can take place.
Activation energy
∆H
•Sufficient atoms or fragments of molecules must be freed for the new bonds to begin forming.
•Once started, the energy released as new bonds are formed causes the reaction to continue.
Activation energy
•All reactions require some activation energy.
•For the reaction of sodium or potassium with water the activation energy is low, and there is enough energy available from the surroundings at room temperature for the reaction to begin spontaneously.
Activation energy
•Reactions can be thought of as the result of collisions between atoms, molecules or ions.
•In many of these collisions, the colliding particles do not have enough energy to react, and just bounce apart, rather like ‘dodgem cars’.
Activation energy
•A chemical reaction will only happen of the total energy of the colliding particles is greater than the required activation energy of the reaction.
Activation energy
•A chemical reaction will only happen of the total energy of the colliding particles is greater than the required activation energy of the reaction.
7.2 Rate of reaction
What does rate of reaction mean?What does rate of reaction mean?The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.
What is the rate of these reactions?
The speed of a reaction is called the rate of the reaction.
rusting baking explosion
slow fast very fast
Rates of reactionRates of reactionWhy are some reactions faster than others?
Reactions, particles and collisionsReactions, particles and collisions
Reactions take place when particles collide with a certain amount of energy.
The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction.
The rate of a reaction depends on two things:
the frequency of collisions between particles
the energy with which particles collide.
If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.
Changing the rate of reactionsChanging the rate of reactions
increased temperature
increased concentration of dissolved reactants, and increased pressure of gaseous reactants
increased surface area of solid reactants
use of a catalyst.
Anything that increases the number of successful collisions between reactant particles will speed up a reaction.
What factors affect the rate of reactions?
Slower and slower!Slower and slower!Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.
As the reaction progresses, the concentration of reactants decreases.
This reduces the frequency of collisions between particles and so the reaction slows down.
percentage completion of reaction
100%0% 25% 50% 75%
reactants
product
Graphing rates of reactionGraphing rates of reaction
ReactantReactant––product mixproduct mix
How can rate of reaction be How can rate of reaction be measured?measured?
Measuring the rate of a reaction means measuring the change in the amount of a reactant or the amount of a product.
What can be measured to calculate the rate of reaction between magnesium and hydrochloric acid?
The amount of hydrochloric acid used up (cm3/min). The amount of magnesium chloride produced
(g/min). The amount of hydrogen product (cm3/min).
+magnesiumhydrochloric
acid+
magnesiumchloride hydrogen
Setting up rate experimentsSetting up rate experimentsWhat equipment is needed to investigate the rate of hydrogen production?
gas syringe
rubber bung
rubber connecterglass tube
conicalflask
magnesium
hydrochloricacid
hydro
gen p
roduce
d (
cm3)
time (seconds)10 20 30 40 50
10
20
30
40
50
60
70
00
x
y
Calculating rate of reaction from graphsCalculating rate of reaction from graphs
rate of reaction =
x
y
rate of reaction =
20 s
45 cm3 rate of reaction = 2.25 cm3/s
The gradient of the graph is equal to the initial rate of reaction at that time
How can the rate of reaction be calculated from a graph?
The reactant/product mixThe reactant/product mix
Collisions and reactions: Collisions and reactions: summarysummary
Temperature and collisionsTemperature and collisionsHow does temperature affect the rate of particle collision?
Effect of temperature on rateEffect of temperature on rateThe higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10 °C causes the rate of reaction to approximately double.
Why does increased temperature increase the rate of reaction?
At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles.
When the particles collide, they do so with more energy, and so the number of successful collisions increases.
Temperature and particle Temperature and particle collisionscollisions
Temperature and batteriesTemperature and batteriesWhy are batteries more likely to rundown more quickly in cold weather?
At low temperatures the reaction that generates the electric current proceeds more slowly than at higher temperatures.
This means batteries are less likely to deliver enough current to meet demand.
How does temperature affect rate?How does temperature affect rate?
The reaction between sodium thiosulfate and hydrochloric acid produces sulfur.
Sulfur is solid and so it turns the solution cloudy.
How can this fact be used to measure the effect of temperature on rate of reaction?
hydrochloric
acid
sodium
chloride
sulfur
sodiumthiosulfa
te
+ + water
sulfurdioxid
e
++
Na2S2O
3
(aq)
2HCl
(aq)
2NaCl
(aq)
S(s)
++ SO2
(g)H2O(l)
+ +
The effect of temperature on rateThe effect of temperature on rate
Effect of concentration on rate of reactionEffect of concentration on rate of reactionThe higher the concentration of a dissolved reactant, the faster the rate of a reaction.
Why does increased concentration increase the rate of reaction?
At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react.
higher concentrationlower concentration
Concentration and particle Concentration and particle collisionscollisions
The effect of concentration on rateThe effect of concentration on rate
Effect of pressure on rate of Effect of pressure on rate of reactionreaction
The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.
Why does increasing the pressure of gaseous reactants increase the rate of reaction?
As the pressure increases, the space in which the gas particles are moving becomes smaller.
lower pressure higher pressure
Effect of surface area on Effect of surface area on rate of reactionrate of reactionAny reaction involving a solid can only take place at the surface
of the solid.
If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?
The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.
This means that there is an increased area for the reactant particles to collide with.
low surface area high surface area
Surface area and particle Surface area and particle collisionscollisions
Reaction between a Reaction between a carbonate and acidcarbonate and acidMarble chips are made of calcium carbonate. They react with
hydrochloric acid to produce carbon dioxide.
The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.
hydrochloric
acid
calcium
chloride
calciumcarbonat
e
+ + water
+ carbon
dioxideCaCO3
(aq)2HCl(aq)
CaCl2
(aq) ++ H2O(aq) +
CO2
(g)
The effect of surface area on rateThe effect of surface area on rate
reaction (time)
en
erg
y (
kJ)
What are catalysts?What are catalysts?Catalysts are substances that change the rate of a reaction without being used up in the reaction.
Catalysts never produce more product – they just produce the same amount more quickly.
Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea).
Ea withcatalyst
Ea withoutcatalyst
Everyday catalystsEveryday catalysts Nickel is a catalyst in the production of margarine
(hydrogenation of vegetable oils).
Many catalysts are transition metals or their compounds. For example:
Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.
Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).
Catalysts in industryCatalysts in industry
Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes.
Why are catalysts so important for industry?
Products can be made more quickly, saving time and money.
Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.
GlossaryGlossaryactivation energy – The amount of energy needed to start a reaction.
catalyst – A substance that increases the rate of a chemical reaction without being used up.
concentration – The number of molecules of a substance in a given volume.
enzyme – A biological catalyst.
rate of reaction – The change in the concentration over a certain period of time.
AnagramsAnagrams
Rates of reaction: summaryRates of reaction: summary
Multiple-choice quizMultiple-choice quiz
Surface areaSurface area• If we make the
pieces of the reactants smaller we increase the number of particles on the surface which can react.
• This makes the reaction faster.
The particles on the surface can react
When cut into smaller pieces the particles on the inside can react
How do we make the How do we make the reaction go faster?reaction go faster?
• There are four things that we can change to make the reaction go faster.
• They are • Temperature• Surface area• Concentration• Using a catalyst
TemperatureTemperature
•When we increase the temperature we give the particles energy
•This makes them move faster •This means they collide with
other particles more often•So the reaction goes faster.
ConcentrationConcentration• If we make one reactant
more concentrated (like making a drink of orange squash more concentrated)
• There are more particles in the same volume to react
• So the reaction goes faster.
• Click here to complete exercise
There are less red particles in the same volume so there is less chance of a collision
There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster
Using a catalystUsing a catalyst
• A catalyst is a chemical which is added to a reaction.
• It makes the reaction go faster.• The catalyst does not get used
up in the reaction.• It gives the reaction the energy
to get started
Endothermic Reaction:
- system absorbs E
- more energy needed to break bonds than released by creating bonds
- change in enthalpy is positive
Exothermic Reaction:
- system releases E
- more energy released by creating bonds than needed to break bonds
- change in enthalpy is negative
100
Cold and hot packsCold and hot packs
• How do instant hot and cold packs work?
101
Hot packHot pack
• Pressing the bottom , the diaphragm breaks.• Calcium chloride dissolves in water and warms
it.• The beverage gets warm.
102
EnergySurroundings
Exothermic processExothermic process• Heat flows into the surroundings
from the system in an exothermic process.
Temperature rises
Hot pack
103
Hot packHot pack
• We will repeat the process in a beaker with calcium chloride ( 25 g CaCl2) + water (25 ml) and a thermometer.
• We will record the initial temperature of the water and the temperature after the dissolution of the salt.
• Observation:• a temperature rise of…….
104
Cold packCold pack
• Water and ammonium nitrate are kept in separate compartments.
• Pressing the wrapper, the ammonium nitrate dissolves in water and absorbs heat.
• The pack becomes cold.
It is used to treat sports injuries.
105
Cold pack
Energy
Surroundings
Endothermic processEndothermic process
• Heat flows into the system from the surroundings in an endothermic process.
Temperature falls
106
Cold packCold pack
• We will repeat the process in a beaker with ammonium nitrate ( 25 g NH4NO3) + water (25 ml) and a thermometer.
• We will record the initial temperature of the water and the temperature after the dissolution of the salt.
• Observation:• a temperature drop of…….
Exothermic ReactionExothermic ReactionReactants Products + Energy 10 energy = 8 energy + 2 energy
Reactants
Products
-HEn
erg
y
Energy of reactants
Energy of products
Reaction Progress
Endothermic ReactionEndothermic ReactionEnergy + Reactants Products
+H Endothermic
Reaction progress
En
erg
y
Reactants
ProductsActivation Energy
Effect of Catalyst on Reaction Effect of Catalyst on Reaction RateRate
reactants
products
En
erg
y
activation energy for catalyzed
reaction
Reaction Progress
No catalyst
Catalyst lowers the activation energy for the reaction.What is a catalyst? What does it do during a chemical reaction?
Basic RevisionBasic Revision• For a chemical reaction to occur,
bonds must be both formed and broken
• Fe + CuSO4 Cu + FeSO4
Basic RevisionBasic Revision• Chemical reactions always involve
energy changes.
• Making and breaking bonds involves energy changes
ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS
Exothermic reactions• These reactions give out heat
energy.• Combustion is an exothermic
reaction.
ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS
Endothermic reactions• These reactions take in heat energy
from their surroundings.• These cause temperatures to fall.
ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS
• Use the apparatus as shown.
• Work with 20ml of each substance in turn.
• Copy the results table before you start
ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS
Reaction temp. before mixing (0C)
temp. after mixing (0C)
Exothermic or Endothermic?
sodium hydroxide sol’n + hydrochloric acid
sodium hydrogen carbonate sol’n + citric acid
copper sulphate sol’n + magnesium powder
sulphuric acid + magnesium ribbon
barium chloride + sodium sulphate
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The Energy of Physical, Chemical and Nuclear The Energy of Physical, Chemical and Nuclear Reactions Reactions
• Thermodynamics - the study of energy and energy transfer
• Thermochemistry - the study of energy involved in chemical reactions
Energy (symbol: E – unit: J joules) Law of Conservation of Energy
- total energy of the universe is constant
- energy can neither be created nor destroyed Δ universe=0 𝐸- energy can be transferred from one substance to another
- energy can be converted into various forms
System - part of the universe that is being studied and
observed (reactants & products)
Surroundings - everything else in the universe
- part of the universe that is likely to be affected by
energy changes in the system
Universe = System + Surroundings Δ𝐸univeres= Δ𝐸system+Δ𝐸surrounding = 0
Any change in the system is accompanied by an equal
and opposite change in the surroundings. Δ𝐸system =−Δ𝐸surrounding
Heat (symbol: Q – unit: J joules) - transfer of kinetic
energy (Ek) spontaneously from warmer to cooler
objects
Temperature (symbol: T – unit: K Kelvin, °C
Celsius degrees) TK = T°C + 273.15
- measure of the average kinetic energy of the
particles that make up a substance or system
Celsius scale – relative to water (0°C water melting
point, 100°C water boiling point)
Kelvin scale – absolute scale (O K is temp. when a
substance has no kinetic energy)
Enthalpy (symbol: H – unit: J joules)
- total internal energy of a substance at a constant pressure
- hard to measure; includes (1) energy of physical state, (2)
energy in bonds, (3) energy in nucleus; includes:
- moving electrons within atoms
- vibration of atoms connected by chemical bonds
- rotation and translation of molecules
- nuclear potential energy of protons and neutrons in atomic
nuclei
- electronic potential energy of atoms connected by chemical
bonds
- easily measure and study change in enthalpy (ΔH) in a
reaction
- enthalpy change of a process is equivalent to its heat change
at constant pressure
- chemical bonds are sources of stored energy
- breaking a bond is a process that requires energy
- creating a bond is a process that releases energy
Representing Enthalpy Changes Representing Enthalpy Changes
• ΔHrxn enthalpy of reaction (depends on
temperature and pressure)
• ΔH°rxn standard enthalpy of reaction at
SATP (25°C, 100 kPa)
(1) Thermochemical Equation (1) Thermochemical Equation – balanced – balanced equation that indicates enthalpy change equation that indicates enthalpy change
(2) Separate Expression (2) Separate Expression
(3) Enthalpy Diagram (3) Enthalpy Diagram
1. Physical changes - Small change in enthalpy (tens of kJ/mol) - Change in intermolecular bonds between molecules as they change phase o ΔHvap enthalpy of vaporization (liquid gas) o ΔHcond enthalpy of condensation (gas liquid) o ΔHmelt enthalpy of melting (solid liquid) o ΔHfre enthalpy of freezing (liquid solid)
o ΔHsoln enthalpy of solution (solid aqueous – dissolving)
2. Chemical changes
- Moderate change in enthalpy (hundreds of kJ/mol)
- Change in intramolecular bonds within molecules as reactants break apart to form products o ΔHcomb enthalpy of combustion
o ΔHneut enthalpy of neutralization
o ΔHf enthalpy of formation
3. Nuclear changes
- Enormous change in enthalpy (billions of kJ/mol)
- Change in nuclear binding energy holding together the nucleus of an atom o A significant amount of mass of the reactants is actually converted to energy
o Einstein: E=mc2, where m=mass and c=speed of light (3.0x108m/s) so a tiny mass is equivalent to a significant amount of energy
Mass Defect o difference in mass between a nucleus and its nucleons (particles found in the nucleus)
o caused by the energy associated with the strong force that holds a nucleus together
o the higher the nuclear binding energy, the more stable the nucleus
Nucleus + Nuclear binding energy Nucleons
References• McGraw-Hill Ryerson Chemistry 12• Nelson Chemistry 12
Rates of Rates of Reactions and Reactions and
EnzymesEnzymes
Rates of ReactionRates of ReactionChemical reactions occur when different atoms or molecules collide:
For the reaction to happen the particles must have a certain amount of energy – this is called the ACTIVATION ENERGY.
The rate at which the reaction happens depends on four things:
1) The temperature of the reactants,
2) Their concentration
3) Their surface area
4) Whether or not a catalyst is used
Measuring rate of reactionMeasuring rate of reactionTwo common ways:
1) Measure how fast the products are formed
2) Measure how fast the reactants are used up
Rate of reaction graphRate of reaction graph
Amount of product formed
Time
Slower reactionFast rate
of reaction here
Slower rate of reaction here due to reactants being used
up
Enzymes are biological catalysts. They help the reactions that occur in our bodies by controlling the rate
of reaction.
Enzymes are denatured
beyond 40OC
EnzymesEnzymes
Yeast is an example of an enzyme. It is used to help a process called fermentation:
Sugar Alcohol + carbon dioxide
The alcohol from this process is used in making drinks and the carbon dioxide can be used to make bread rise.
Enzymes work best in certain conditions:
Enzyme activity
Temp pH pH400C
Could be protease (found in the stomach)
Could be amylase (found in the intestine)
Uses of enzymesUses of enzymes1) Enzymes are used in washing powders to help digest food stains. Biological washing powders will only work on 400C or lower.
2) Enzymes are used in baby foods to “pre-digest” the proteins.
3) Enzymes are used to convert starch into sugar which can then be used in food.
4) Conversion of glucose into fructose – glucose and fructose are “isomers” (they have the same chemical formula), but fructose is sweeter.